Method for preparing an oriented-porosity dielectric material on a substrate by means of electromagnetic and/or photonic treatment

ABSTRACT

Provided a method for producing an oriented-porosity dielectric material on a substrate. The method includes depositing a vapor phase on a substrate of a composite layer comprising a material forming a matrix and a compound comprising chemical groups capable of being oriented under the effect of an electromagnetic field and/or photonic radiation; treating the composite layer to obtain the cross-linking of the material forming a matrix; and subjecting the substrate coated with the composite layer to an electromagnetic field and/or a photonic radiation.

FIELD OF THE INVENTION

The invention relates to a method for preparing an oriented-porositydielectric material, deposited on a substrate, said materialconventionally consisting of a thin layer, i.e. a layer having athickness less than 10 μm.

In particular, the invention relates to a method for preparing anorganised-porosity and low-permittivity dielectric material, i.e.generally displaying a dielectric constant k less than 3.9, and morespecifically a very low permittivity material, referred to as “ultralow-k” (2.2<k<3) and “extreme low-k” (1.5<k<2.2), said material beingdeposited on a substrate.

This method finds applications in the field of microelectronics,particularly in the production of materials in the form of thin layersusable in interconnections in order to isolate electrically theintermetallic lines serving to carry the electric signal and preventparasite capacitances therebetween, and to create any type of circuitmaking use of the separating and isolating function of thin layers.

This method also finds applications in the production of separatingmembranes, diffusion membranes, molecular detectors, optical materials,polymer masks.

STATE OF THE RELATED ART

The miniaturisation of microelectronic components along with theincrease of the performance of same have seen considerable developmentin recent years. This progress has been accompanied by the developmentof new materials capable of replacing the materials conventionally usedand wherein the properties are rendered insufficient by saidminiaturisation. In particular, the need to reduce parasite capacitancesbetween the interconnection lines incites the use of new dielectricmaterials displaying very low permittivity.

One of the solutions selected to date has been to use porous dielectricmaterials, the presence of these pores helping reduce the permittivityof the material, due to the air present therein.

For this purpose, it was particularly proposed to deposit, on areasintended to be protected by a porous dielectric material, a compositematerial comprising, on one hand, a dielectric material forming a matrixand, on the other hand, a compound, conventionally of the polymer type,dispersed in the matrix, and, after cross-linking of the matrix,extracting said compound so as to replace the sites occupied by same inthe matrix by pores filled with air.

The compound dispersed in the matrix, wherein the only function is toenable the formation of pores within said matrix is referred to as a“pore-forming compound”.

The composite material may be deposited by means of a solution processsuch as spin-coating and dip-coating or by means of a plasma process,such as the PECVD (Plasma Enhanced Chemical Vapour Deposition)technique.

The pore-forming compound is conventionally extracted by means of a heattreatment at a suitable temperature to obtain the decomposition of thepore-forming compound under the effect of heat and, concomitantly, thedesorption thereof from the matrix. Some research also demonstrates thatthe use of ultraviolet irradiation combined with thermal annealingenables improved decomposition of the pore-forming compound.

However, this type of treatment, conventionally used at the presenttime, generates a porous material displaying amorphous and randomporosity and, for this reason, high mechanical weakness. Therefore, itis difficult to incorporate this type of material in microelectronicprocesses requiring, for example, a chemico-mechanical polishing step,as these materials do not display a sufficient mechanical resistance.

In order to obtain a more regular porosity than the materials obtainedwith a polymer type pore-forming compound described above, Grosso et al.(J. Mater. Chem, 10 (2000), 2085) proposed to prepare a porousdielectric material:

-   -   by depositing on a silicon substrate by means of dip-coating of        a layer of solution obtained by mixing an ethanol solution        containing a silicon-based precursor: tetraethoxysilane, and a        surfactant compound;    -   by means of heat treatment of the deposited layer so as to        obtain cross-linking thereof into silica and removal of the        surfactant compound.

The advantage of this method is that it makes it possible to obtain bymeans of a solution process a layer of porous dielectric materialdisplaying a specific periodicity. However, this method cannot beintegrated in a microelectronic integration process. In fact, inmicroelectronics, methods in solution are not usable as they causeadherence, mechanical resistance and thermal budget problems.

It is important to note that the prior art does not include, at thepresent time, any example of an embodiment of structured-porosity thinlayers deposited in vapour phase (such as CVD or PECVD, in particular).

Therefore, there is a genuine need for a preparation method which makesit possible to obtain an oriented-porosity dielectric material by meansof vapour phase deposition displaying a periodicity of the properties ofsame. In particular, there is a genuine need for a porous dielectricmaterial, generally, in the form of a thin layer, displaying astructured, homogenous and organised porosity displaying, for thisreason, a greater mechanical resistance than that of the materialsobtained with the methods of the prior art and which are able to retaintheir physical integrity during subsequent treatments, such aschemico-mechanical polishing.

DESCRIPTION OF THE INVENTION

Therefore, the present invention relates to a method for producing anoriented-porosity dielectric material on a substrate comprising:

a) vapour phase deposition on a substrate of a composite layer,comprising a material forming a matrix and a compound comprisingchemical groups capable of being oriented under the effect of anelectromagnetic field and/or photonic radiation;

b) the treatment of the composite layer to obtain the cross-linking ofthe material forming a matrix;

said method also comprising a step c) consisting of subjecting saidsubstrate coated with said composite layer to an electromagnetic fieldand/or a photonic radiation, said step c) being carried outsimultaneously with step a), when said layer is subjected to a photonicradiation or carried out before and/or simultaneously with step b), whensaid layer is subjected to an electromagnetic field, given that theelectromagnetic field is applied in contact with the substrate, saidstep c) being carried out so as to align the chemical groups mentionedin step a) in a predetermined direction.

In this way, the innovative nature of said method according to theinvention essentially lies in the combination of step a) and step c), inthat these steps will help favour an alignment of the molecules ofcompounds, which will be capable of being arranged in a preferreddirection according to the interference applied, due to the nature ofthe specific chemical groups comprised by said compounds. Said compoundsaligned in a preferred direction will help give the material aperiodicity of the properties inherent to the nature of these compounds.

According to the invention, the step consisting of placing the substratecoated with the layer in contact with an electromagnetic field and/or aphotonic radiation may be carried out in different ways.

In this way, said step c) may be performed before step b), i.e. forexample, either simultaneously with the deposition step a), which ispossible when the treatment is carried out by means of photonicradiation or by means of an electromagnetic field, or once thedeposition step a) has been completed, which is only possible when thetreatment is carried out by means of an electromagnetic field, giventhat it is preferable, in both cases, to implement said step c)simultaneously with the deposition step a). In fact, in the latter case,the electromagnetic field and/or the photonic radiation will enable analignment of the molecules of compounds from the deposition thereof onthe substrate and will enable a preferential arrangement thereofthroughout the thickness of the deposited layer. Moreover, when step c)is carried out simultaneously with step a), the composite material is,generally, cross-linked to a low degree, which enables an improvedorientation of the abovementioned compounds. For this purpose, thecomposite material may be deposited under mild conditions (such as apressure P>2 torrs, a temperature less than or equal to 300° C. and apower of less than 500 Watts). For example, when the material issilica-based, more than 1% —SiOH groups should be advantageouslyincorporated in the layer (where it is possible to verify this contentby means of infrared analysis).

As mentioned above, the treatment by means of photonic radiation may beimplemented only simultaneously with the deposition step a), to enable aprogressive orientation of the compounds capable of being aligned. Infact, once the layer has been deposited in the vapour phase, thephotonic radiation, if applied after the deposition step, would nolonger be capable of inducing an orientation of the molecules, due tothe inertia of the molecules in a network subject to stress and the weakradiation-material interaction. It is also noted that a distinction ismade between a photonic radiation used for degradation and the photonicradiation according to the invention, which is implemented in this casewith no thermal annealing step (due to the fact that the radiation takesplace simultaneously with deposition), in order to prevent thedegradation of the compounds capable of being oriented during theapplication of the photonic radiation.

For the treatment with an electromagnetic field, said step c) may beperformed simultaneously with step a), after said step a) but beforestep b) and/or simultaneously with said step b).

Finally, it can also be envisaged to implement step c) both before stepb) and simultaneously with said step.

Irrespective of the selected alternative with respect to the timesequence of step C), the electromagnetic field applied is,advantageously, according to the invention, an electric voltage applieddirectly to the substrate coated with the layer. Preferentially, theelectric voltage applied is a direct voltage, such that the compoundmolecules are arranged according to a given phase in the sameirreversible manner, i.e. always in the same direction imposed by theelectric field. The electric voltage, preferentially direct voltage, maybe between 1 V and 100 V. It may be envisageable, depending on thenature of the compound used, to change the voltage by 1 V steps until aworking point is found. The working point corresponds to the voltage forwhich the compound molecules are arranged in an invariable manner.

The photonic radiation may consist of ultraviolet radiation. Dependingon the nature of the chemical groups of the compound capable of beingoriented under the effect of UV radiation, it will be advisable to set:

-   -   the wavelength of the ultraviolet radiation, such that the        groups capable of being oriented can absorb this radiation and        be oriented once the absorption has been completed;    -   the radiation intensity such that it is in agreement with the        absorption spectrum of the suitable groups of the compound.

The radiation intensity may vary, for example, between 1 and 5 eV. Itmay be envisageable, according to the photosensitive groups present onthe compound, to vary the wavelength of the UV source gradually in orderto find a working point.

The method according to the invention is applied to the preparation ofan organised-porosity dielectric material, whereby the material forminga matrix will be advantageously a dielectric material and the compoundcomprising the chemical groups capable of being oriented under theeffect of an electromagnetic field and/or a photonic radiation will be adecomposable pore-forming compound, said compound preferentially notbeing capable of being organised in liquid crystal form.

In this case, the method will also comprise a step d) consisting oftreating the composite layer to obtain a decomposition of thepore-forming compound and the extraction thereof.

Therefore, the pores resulting from the removal of these pore-formingcompounds will be oriented, in a homogeneous manner, and in a preferreddirection, due to the fact that they inherit the position of themolecules of homogeneous compounds after the removal thereof.

In this case, step c) consisting of subjecting said substrate coatedwith said layer to a photonic radiation will always be performedsimultaneously with step a).

In the case of a treatment by means of an electromagnetic field, step c)may be performed before steps b) and/or d), i.e. for example, eithersimultaneously with the deposition step a), or once the deposition stepa) is complete, given that it is preferable to implement said step c)simultaneously with the deposition step a). In fact, in the latter case,the electromagnetic field will enable an alignment of the molecules ofpore-forming compounds from the deposition thereof on the substrate anda preferential arrangement thereof throughout the thickness of thedeposited layer.

In an alternative embodiment, for the treatment by means of anelectromagnetic field, said step c) may be performed simultaneously withstep b) and/or d).

Finally, it can also be envisaged to implement step c) both before thetreatment steps b) and/or d) and simultaneously step b).

According to the invention, the dielectric material forming the matrixmay be any material, preferentially silicon-based, known to displaydielectric properties.

It may consist of silicon Si, SiO₂, SiOC, silicon carbide (SiC),hydrogenated silicon carbide (SiC:H), silicon oxynitride (Si:O:N),silicon nitride (SiN), silicon carbonitride (Si:CN), fluorinated glass(SiOF), organofluorosilicates (Si:O:C:H:F), organosilicate compounds(SiOCH), borosilicate glass (Si:OBHP), where BHP indicates boron,hydrogen, phosphorus, respectively.

It may also consist of a carbonaceous material, such as a crystallisedcarbon material according to the specific crystalline system fordiamonds (frequently called DLC carbon).

Advantageously, the deposition may be performed by means of PlasmaEnhanced Chemical Vapour Deposition (PECVD). This technique isparticularly suitable when the material forming the matrix is a materialof the type SiO₂, SiOC or SiOF.

Other methods may be used in addition to PECVD, such as high-density:PECVD (ADP), photon-enhanced CVD (particularly in the case of the useof the photonic radiation for orientation), cryogenic CVD,filament-enhanced CVD.

The substrates used according to the invention may be GaAs (galliumarsenide), SiGe alloy, silicon boronitride semiconductor substrates,substrates containing silicon such as epitaxial silicon, silicon dioxideSiO₂, silicon carbides (SiC), SiCN, the materials SiOCH, SiOCF.

In the case of PECVD deposition, a source of dielectric materialintended to form the matrix and a source of compound capable of beingoriented are jointly or successively injected in the form of vapour intothe confinement of a plasma reactor. In the case of precursors in liquidform, they are heated to a temperature above their boiling pointbeforehand. In this type of technique, the sources may be split, i.e.several sources of dielectric material and possibly several sources ofcompounds capable of being oriented may be used. Once the sources havebeen injected, a voltage is applied in the form of a difference inpotential, generally between two parallel electrodes placed in a vacuumconfinement, so as to generate a plasma discharge. It is specified thatthe term plasma refers to a gaseous medium consisting of free electrons,ions and atoms or neutral molecules, said particles being in proportionssuch that the medium is macroscopically neutral from en electrical pointof view. The plasma initiating source may be, for example, aradiofrequency voltage. The plasma state particularly makes it possible,depending on the electromagnetic power injected, to retain or split themolecules of the precursors injected.

The molecules forming the matrix and/or the compounds capable of beingoriented should be able to be injected into a vacuum plasma confinement,in vapour (gas) form. These injections may be implemented by means ofvarious methods, such as the helium thrust method. The molecules usedshould advantageously have a saturating pressure value compatible withvaporisation and injection in vapour form.

A substrate, for example, made of silicon, connected to one of theelectrodes, serves as a deposition substrate. The fact that saidsubstrate is connected to an electrode makes it possible to impose atemperature on said substrate, so as to facilitate the deposition on thesubstrate, said temperature possibly ranging from ambient temperature toa temperature of 300° C. It may be possible to envisage increasing thetemperature by 25° C. steps. Under the effect of gravitation and thediffusion of the species in the plasma, the molecules from the precursorof the matrix and the compound will be jointly adsorbed on the surfaceof the substrate and therefore increase the deposition.

According to the invention, the pore-forming compound deposited shouldcomprise groups capable of being oriented under the effect of theelectromagnetic field and/or a photonic radiation and, in addition,should be able to be decomposed during step d). The term decomposablerefers to a compound liable to lose its physical integrity under theeffect of a disturbance, for example thermal, and which may bedecomposed into volatile substances, thus facilitating the extraction ofsame. Said pore-forming compound may advantageously be decomposed duringstep d) by means of a heat treatment by heating it to a temperaturegreater than or equal to the decomposition temperature thereof, whichvolatilises and extracts the pore-forming compound molecules and enablesthe formation of pores within the dielectric material forming thematrix. The temperature greater than or equal to the decompositiontemperature of the pore-forming compound may be obtained by heating thesubstrate to the suitable temperature, the substrate acting as a heatingplate, said heating possibly taking place either in the depositionreactor, or in an oven separate from the deposition reactor. Thoseskilled in the art will be able to achieve such a temperature, forexample, in 25° C. steps. Besides heating on a heating plate, thethermal decomposition of the pore-forming compound may be obtained bymeans of exposure to radiation, mechanical energy or by means ofparticle irradiation.

Besides the thermal decomposition, the treatment according to step d)may be envisaged by means of solvent extraction (for example withsupercritical CM or by means of chemical etching (for example by meansof hydrogen plasma).

The pore-forming compound decomposed in this way diffuses outside thedielectric material forming the matrix or diffuses inside the dielectricmaterial, forming in both cases unoccupied, empty zones, referred to aspores, which make it possible to reduce the permittivity of thedielectric material forming the matrix.

Advantageously, the molar ratio (pore-forming compound/dielectricmatrix) is less than or equal to 0.5.

Advantageously, the pore-forming compound is a polymer. In this way,while decomposing, it may generate pores with a tubular structure, dueto the vacant space left by the polymer chains.

As mentioned above, the pore-forming compound comprises groups capableof being oriented under the effect of an electromagnetic field,particularly an electric field, or a photonic radiation.

Groups liable to be oriented under the effect of an electromagneticfield may be polar groups that can be selected from the —OH(particularly —SiON), —CN groups, the heterocyclic groups, —CO₂H,esters, —SiC, —SiF, —SiNH, —CF, —NH, —F groups.

In this way, the pore-forming compound may be advantageously a polymerobtained by means of polymerisation of at least one ethylenic monomercomprising a polar group selected from:

a) ethylenic monomers comprising one or more —CO₂H groups and the estersthereof and ethylene monomers comprising one or more CN groups, such as:

-   -   (meth)acrylics, possibly substituted, such as acrylic acid,        methacrylic acid or crotonic acid;    -   alkyl and alkylene (meth)acrylates such as methyl, ethyl,        propyl, butyl, octyl, 2-ethyl-hexyl, cyclohexyl or 2-hexene        (meth)acrylates and the derivatives thereof obtained by        substituting said alkyls and alkylenes such as methoxyethyl,        ethoxyethyl, ethoxypropyl, hexafluoroisopropyl, 2-hydroxyethyl,        2- or 3-hydroxypropyl, 2,3-dihydroxypropyl, polyethoxyethyl or        polyethoxy-propyl (meth)acrylates;    -   aryl (meth)acrylates such as phenyl or benzyl (meth)acrylates,        and the derivatives thereof obtained by substituting said aryls;    -   poly(ethylene glycol) or poly(propylene glycol) (meth) acrylates        and di(meth)acrylates;    -   (meth)acrylamides and the N-substituted derivatives thereof such        as N-methylacrylamide, N,N-dimethylacrylamide,        N,N-dimethylmethacrylamide,        2-acrylamido-2-methyl-1-propanesulphonic acid,        N-[3-(dimethylamino)propyl] acrylamide or 2-(N,N-diethylamino)        ethylmethacrylamide;    -   unsaturated dicarboxylic acids such as maleic acid, fumaric acid        or itaconic acid, and the esters thereof such as dimethyl        maleate, dimethyl fumarate or diethyl fumarate;    -   nitrile monomers such as acrylonitrile;

b) ethylenic monomers comprising a heterocyclic group such as a pyridinecycle (for example, 2-vinylpyridine, 4-vinylpyridine,2-methyl-5-vinylpyridine, 4-methyl-5-vinylpyridine orN-methyl-4-vinylpyridine), a piperidine cycle (for example,N-methyl-4-vinylpiperidine), an imidazole cycle (for example,2-methyl-1-vinylimidazole), a pyrrolidone cycle (for example,N-vinyl-pyrrolidone) or a pyrroledione cycle (for example, maleimide);

c) ethylenic monomers comprising an —O—CO—R group where R represents analkyl group (for example, vinyl acetate or vinyl propionate), or an arylgroup (for example, vinyl benzoate);

d) ethylenic monomers comprising an —OR group, known as vinylethers,where R represents an alkyl group (for example, methylvinylether), saidgroup possibly comprising one or more oxygen atoms (for example,ethoxyethyl-vinylether) or one of more amino groups (for example,dimethylaminoethylvinylether); and

e) ethylenic monomers comprising a —C(O)R group where R represents analkyl group (for example, vinylmethylketone)

and, if applicable, one or more monomers selected from:

-   -   ethylenic monomers comprising an exclusively hydrocarbonate        aromatic group such as styrene or α-methylstyrene;    -   exclusively hydrocarboneous ethylenic monomers such as 1-hexene,        norbornene or acenaphthylene.

In particular, polymers liable to form pore-forming compounds capable ofbeing oriented under the effect of an electromagnetic field are(acrylonitrile/butadiene) copolymers, (styrene/acrylonitrile)copolymers, polyvinylpyrrolidines, cyclodextrins.

Groups liable to be oriented under the effect of a photonic radiationmay be —N═N— azo groups.

Pore-forming compounds meeting this criterion are linear polymersobtained by means of photopolymerisation of monomers such as azobenzenesor cross-linked polymers obtained by means of photopolymerisation ofprepolymers comprising groups liable to be cross-linked (referred to ascross-linking groups) and monomers, oligomers or polymers comprisinggroups capable of reacting with cross-linking groups, the resultingpolymer comprising photo-orientable groups.

According to the invention, step b) and step d) may be implementedsimultaneously.

The treatment step b) intended to induce the cross-linking of thedielectric material forming the matrix generally consists of a heatingof the substrate to an effective temperature to induce cross-linking. Itis specified that the term cross-linking refers to a reaction inducing aconnection of the chains of the matrix with each other via bridges orchemical bonds, in order to form a network displaying superiorphysicochemical properties, such as rigidity, to the layer of materialinitially deposited.

Step d) is advantageously performed by heating to an effectivetemperature to obtain a decomposition of the pore-forming compound.

If said step b) is implemented simultaneously with step d), the heatingwill make it possible to decompose, concomitantly, the pore-formingcompound. The heating means to cross-link the dielectric material may besimilar to those described above for the decomposition of thepore-forming compound. Said heating step of the dielectric materialforming the matrix with a view to cross-linking same, may be termed“annealing”, where said annealing may be assisted by ultravioletradiation, an electron beam, by means of H₂ plasma, in supercriticalfluid (such as CO₂).

Step b) and step d) should be performed at an effective temperature,where said effective temperatures may be determined easily, so as toobtain the desired purpose, i.e. the cross-linking and/or decompositionof the pore-forming compound.

In particular, the temperatures will be selected, so as to obtain astabilised material advantageously meeting the following criteria:

-   -   satisfactory ageing;    -   humidity absorption of less than 1% (determined by weighing);    -   a stable dielectric constant (deviation of less than 0.05);    -   a satisfactory mechanical resistance, whereby the film should        not delaminate or crack during a mechano-chemical polishing        step, such as mechano-chemical polishing with a basic pH        colloidal silica particle-based suspension (pH of the order of        8);    -   a heat resistance, whereby the film should be stable up to        450° C. (loss of mass less than 5%);    -   a physicochemical inertia, whereby the ellipsometric and stress        measurements should remain constant, and not displaying the        appearance of new adsorbed bonds on the infrared spectrum over        time.

All the steps are advantageously performed at a pressure greater than 5torrs.

In this way, the method according to the invention may make it possibleto obtain an organised-porosity dielectric material, particularly in theform of a thin layer, with low permittivity, i.e. generally displaying adielectric constant k less than 3.9, and more specifically a very lowpermittivity material, referred to as an “ultra low-k” (2.2<k<3) and“extreme low-k” (1.5<k<2.2) material.

In addition to applications in the field of microelectronics, thesematerials may find applications in the field of catalysis, separatingmembranes, chemical sensors and optical interconnections.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS Example 1

A silicon sheet is deposited on a heating substrate.

On said silicon sheet, a layer of SiOC (material forming a matrix)comprising a pore-forming compound is deposited by means ofphoton-assisted PECVD by injecting:

-   -   a organosilicate compound: tetraethoxysilane, a precursor of the        layer of SiOC;    -   a pore-forming compound comprising photosensitive groups: a        diarylethene

according to a pore-forming compound flow rate ratio with respect to theorganosilicate compound of 2.

During the deposition step, a 10 mW ultraviolet radiation at awavelength of 325 nm is applied by irradiating the substrate coated withthe layer, whereby the molecules of the pore-forming compound adsorbedon the surface of the substrate are oriented in a given direction.Finally, the substrate coated with the layer is subjected to heating ata temperature of 425° C. for 1 hour, whereby the pore-forming compoundis removed and the organosilicate compound is cross-linked. In this way,a dielectric layer displaying a regularly arranged porosity is obtained.

1. A method for producing an oriented-porosity dielectric material on asubstrate, the method comprising: a) depositing a composite layer on asubstrate by a vapor phase deposition, wherein the composite layercomprises a material forming a matrix and a compound comprising at leastone chemical group capable of being oriented under the effect of anelectromagnetic field and a photonic radiation; b) treating thecomposite layer to cross-link the material, thereby forming a matrix;said method further comprising c) subjecting said substrate coated withsaid composite layer to an electromagnetic field and a photonicradiation, wherein said subjecting c) is carried out so as to align thechemical groups the deposited composite layer in a predetermineddirection.
 2. The method according to claim 1, wherein said subjectingc) is carried before and/or simultaneously with said treating b), andwherein the electromagnetic field is applied in contact with thesubstrate.
 3. The method according to claim 1, wherein theelectromagnetic field is-an electric voltage applied directly to thesubstrate coated with the layer.
 4. The method according to claim 2,wherein the electric voltage is from 1 V to 100 V.
 5. The methodaccording to claim 1, wherein the material forming a matrix is adielectric material and the compound comprising the at least onechemical group capable of being oriented under the effect of anelectromagnetic field and a photonic radiation is a decomposablepore-forming compound.
 6. The method according to claim 5, furthercomprising d) treating the composite layer to obtain a decomposition ofthe pore-forming compound, thereby extracting the pore-forming compound.7. The method according to claim 5, wherein the dielectric materialforming a matrix is a non-polymeric silicate material.
 8. The methodaccording to claim 7, wherein the dielectric material is at least onesilicon-based material selected from the group consisting of Si, SiO₂,SiOC, SiOF, SiC, a hydrogenated silicon carbide, a silicon oxynitride, asilicon nitride, a silicon carbonitride, fluorinated glass, anorganofluorosilicate compound, an organosilicate compound, andborosilicate glass.
 9. The method according to claim 7, wherein saiddepositing a) is performed by a plasma enhanced chemical vapourdeposition.
 10. The method according to claim 5, wherein thedecomposable pore-forming compound is a polymer.
 11. The methodaccording to claim 10, wherein the polymer is obtained by polymerisationof at least one ethylenic monomer comprising a polar group selected fromthe group consisting of: an ethylenic monomer comprising one or more—CO₂H groups and an ester thereof; an ethylenic monomer comprising oneor more CN groups; an ethylenic monomer comprising a heterocyclic group;an ethylenic monomer comprising an —O—CO—R group, wherein R is an alkylgroup, or an aryl group; an ethylenic monomer comprising an —OR group,wherein R is an alkyl group, and the —OR group may comprise one or moreoxygen atoms or one of more amino groups; and an ethylenic monomercomprising a —C(O)R group, wherein R is an alkyl group; and, optionally,one or more monomers selected from the group consisting of: an ethylenicmonomer comprising exclusively at least one hydrocarboneous aromaticgroup; and exclusively a hydrocarboneous ethylenic monomer.
 12. Themethod according to claim 1, wherein said treating b) is performed byheating to an effective temperature, thereby obtaining cross-linking ofthe material forming the matrix.
 13. The method according to claim 6,wherein said treating d) is performed by heating to an effectivetemperature, thereby obtaining a decomposition of the pore-formingcompound.
 14. The method according to claim 6, wherein said treating b)and said treating d) are performed simultaneously.